378 1
Inorg. Chem. 1989, 28, 3781-3782 can then be formed either by redox reaction of the ArCH202with the POA'" catalyst, or, due to the high termination rate of the arylperoxy radicals, by disproportionation of two ArCH202. In this case the corresponding alcohol is also formed.2O Subsequent oxidation of the latter may also give p-tolualdehyde. In turn, oxidation of the aldehyde produces the corresponding carboxylic acid. At this point, reoxidation of the reduced POA catalyst in the presence of excess oxygen is an easy and fast process, which efficiently closes the catalytic cycle." Further, we note that, after about 20 h of irradiation, the reaction performed either with 1 or 2 showed the presence of a white precipitate (ca.3% of the starting pxylene) that turned out to be analytically pure terephthalic acid. At this point, light scattering and diffusion from the heterogeneous reaction mixture considerably decreases the photocatalytic reaction rate. However, it is noteworthy that, after the first oxidation step, deactivation of the second CH3 by the electron-withdrawing carboxylic group does not prevent complete oxidation of p-xylene. Finally, we point out that recovery of the catalyst, after both the anaerobic and the aerobic reactions, gives partially insoluble products that, although retaining the intact POA structure (IR spectroscopy), contain small amounts of organics strongly bound to the inorganic acid.23 Investigation of this and other aspects of the reported processes are under way and will be reported at a later stage. (23) Preliminary results (IH NMR) indicate that tram of CH,CN oxidation products are present.
Istituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del CNR P.O.Box 10 00016 Monterotondo Stazione, Roma, Italy
Donato Attanasio* Lorenza Suber
Table I. Infrared Spectral Data for the Anions and Dimers 2050 (m) 2015 (s) 1985 (w) Mn2(CO)io 2070 (m) 2040 (m) 1950 (m) 1885 (w) C02(CO)8 1840 (m) 1950(m) 2070 (s) R~~(CO)IO 1906 (m) (PPN)Re(CO) (PPN)MII(CO)~ 1890 (m) (PPN)CpMo(CO), 1893 (s) 1890 (m) (PPN)Co(CO), Cp2M02(C0)6
1910(m) 1775(w) 2010 (m) 1965 (m) 1860 (s) 1860 (s) 1780 (s)
1720(w)
Table 11. Observed Rate Constants for Reaction of Re(CO)< with
M2 M2 Cp2M02(C0)6
M~~(CO)IO
C02(CO)8
[M21/ M 2.0 X 2.00 x 10-2 6.00 X 8.00 X 2.00 X IO-' 1.00 x 10-3 2.00 X 6.00 X 8.00 X 2.00 X 10-1 3.0 X lo-* 6.00 X 8.00 X 1.00 X lo-'
koM: s-l 1.04 X 10-1 f 5.31 x 10-1 f 1.12 f 3.6 X 1.78 f 2.0 X 4.08 f 5.7 X 4.44 x 10-2 f 5.04 X f 9.31 X lo-* f 1.39 X 10-1 f 3.01 X 10-1 f 2.07 f 0.04 2.60 f 0.01 2.70 f 0.02 3.00 & 0.02
1.5 X lo-' 1.2 x 10-3
3.3 3.4 3.3 2.5 1.9
x 10-4 X IO4 X X X
IO-)
'Concentrations are in molarity, [Re(CO) O )C~O ~ ( C O>) ~ Mn2(CO)lo,as shown in Table 111. This dependence does not correlate with the reduction potentials for the dimers, which are also given in Table 111. This casts doubt on an outer-sphere mechanism for the electron transfer. Possible inner-sphere mechanisms requiring dissociation of a ligand from the anion or the dimer are precluded because the electron transfer occurs more rapidly than ligand dissociation. An inner-sphere mechanism involving nucleophilic attack of the anion on the dimer is indicated. Possible sites for nucleophilic attack would be (1) the metal-metal antibonding orbital, (2) the metal center, and (3) a carbon of a carbonyl. Attack at the metal center would seem unlikely since the most sterically constrained metal (seven-coordinate) reacts more rapidly.'~~It is difficult to distinguish between the other two possibilities at this point. On the basis of the known preference for nucleophilic attack at CO's with less electron density and the rapid reaction of C ~ , M O ~ ( C Owe) ~favor ~ ~ attack ' ~ at the carbonyl. Future studies will be directed toward this question.
Acknowledgment. We acknowledge the Department of Energy (Grant ER 13775) for support of this research. We also acknowledge the assistance of Gary Sagerman, Jack Shreves, Matt Michalski, Kevin Kujawa, and Frank Hicklin in construction of the IR stopped-flow spectrometer. Supplementary Material Available: Plots of kobsversus [M,] and of kinetic data for reactions of Re(CO)S- with M2 (9 pages). Ordering
A schematic of the infrared stopped-flow instrument is shown in Figure 1. The infrared source is a tunable carbon monoxide laser (Edinburgh Instruments PL3). The beam passes through an iris diaphragm and the Irtran-2 cell (Wilmad Glass Co., Inc.) and into the HgCdTe detector (Infrared Associates, Inc.). The signal from the detector is processed by a stopped-flow operating system (OLIS). The stopped-flow system is composed of Teflon and KelF components. The syringes are driven by an air cylinder (Power Drive Inc.) through a Berger ball mixer (Research Instruments and Mfg. and Commonwealth Technology Inc.). The stopcocks and tubing are from Hamilton. For M2 = Co2(CO), the plots of kob versus [Co2(C0),] have a substantial intercept. This may indicate a process that is independent of [Co2(C0),] or may be a result of the instability of Co2(CO), in T H F solution. Since the other dimers do not show such an intercept, we favor the latter explanation. Dessy, R. E.; Weissman, P. M.; Pohl, R. L. J . Am. Chem. SOC.1966, 88, 5 1 17.
(a) Wilson, F. C.; Shoemaker, D. P. J . Chem. Phys. 1957, 27, 807. (b) Adams, R. D.; Cotton, F. A. Inorg. Chim. Acta 1973, 7, 153. Zhen, Y . ;Atwood, J. D. J . Am. Chem. SOC.1989, I l l , 1506. Ford, P. C.; Rokicki, A. Adu. Organomet. Chem. 1988, 28, 139.
Preparation and X-ray Structure of (Tetramethyldibenzotetraaza[14]annulene)chromium Dimer, [(tmtaa)Cr],. A Multiply Bonded Complex of Dichromium(I1) without Bridging Ligands Understanding the factors that promote or inhibit the formation of metal-metal multiple bonds remains even today a very challenging and interesting goal.' This is especially puzzling in the chemistry of Cr(I1) where the well-known ability of the d4 electronic configuration to form unusually short Cr-Cr quadruple bonds2 contrasts with theoretical calculations that predict for the quadruple bond either little3 or no contribution4 to the ground state. Furthermore, the occurrence of Cr-Cr quadruple bonds, with the only three exceptions being Cr2Me8Li4,5aCr2(C4H8)4Li4,5b is confined to a and Cr2(PMe3)2(CH2SiMe3)z(p-CH2SiMe3)26 homogeneous series of compounds containing ligands with the characteristic geometry of the bridging three-center chelating systems.'.' In order to identify a possible role of the ligands in forcing the formation of Cr-Cr bonds, we started an extensive study of new classes of Cr(I1) compounds, containing ligands with most diverse geometries.8-10 Herein we report an example where the formation of a Cr-Cr multiple bond without the assistance of bridging atoms is caused by the unique geometry of a macrocyclic ligand. Reaction of M ~ & I - ~ ( L ~ T H Fwith ) ~ ' 2 equiv of tetramethyldibenzotetraaza[ 14lannulene (tmtaaH2) in toluene proceeds smoothly with evolution of methane (Scheme I)" and formation
(1) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Wiley: New York, 1982. (b) Cotton, F. A.; Walton, R. A. MetalMetal Multiple Bonds in Dinuclear Clusters. Struct. Bonding (Berlin) 1985, 62, 1. (2) (a) Cotton, F. A.; Koch, S.; Millar, M. J . Am. Chem. SOC.1977, 99, 7371. (b) Cotton, F. A.; Feng, X.; Kibala, P. A.; Matusz, M. J . Am. Chem. SOC.1988, 110, 2807. (3) (a) Guest, M. F.; Garner, C. D.; Hillier, I. H.; Walton, I. B. J . Chem. SOC.,Faraday Trans. 2 1978, 74,2092. (b) Benard, M. J . Am. Chem. SOC.1978, 100, 2354. (c) Cotton, F. A.; Stanley, G. G. Inorg. Chem. 1977, 16, 2671. (d) Kok, R. A.; Hall, M. B. Inorg. Chem. 1985,24, 1542. (e) Davy, R. D.; Hall, M. B. J . Am. Chem. Soc. 1989,111,1268. (f) Hall, M. B. Polyhedron 1987, 6, 679. (4) (a) Guest, M. F.; Hillier, I. H.; Garner, C. D. Chem. Phys. Lett. 1977, 48, 587. (b) Benard, M. J . Chem. Phys. 1979, 71, 2546. (c) Garner, C. D.; Hillier, I. H.; Guest, M. F.; Green, J. C.; Coleman, A. W. Chem. Phys. Lett. 1976, 41, 91. (d) Benard, M.; Veillard, A. Nouu. J. Chim. 1977.1.97. (e) DeMello, P. C.; Edwards, W. D.; Zerner, M. C. J . Am. Chem. SOC.1982, 104, 1440. ( 5 ) (a) Krausse, J.; Marx, G.; Schiidl, G. J . Organomet. Chem. 1970, 21, 159. (b) Krausse, J.; Schiidl, G. J . Organomet. Chem. 1971, 27, 59. (6) Anderson, R. A,; Jones, R. A.; Wilkinson, G.; Hursthouse, M. B.; Abdul-Malik, K. J. Chem. SOC.,Chem. Commun. 1977, 283. (7) Wilkinson, G.; Gillard, R. D.; McCleverty, J. A. Comprehensiue Coordination Chemistry; Pergamon Press: Oxford, England, 1987; Vol. 3.
(8) Edema, J. J. H.; Gambarotta, S.; van Bolhuis, F.; Spek, A. L. J . Am. Chem. SOC.1989, 111, 2142. (9) Edema, J. J. H.; Gambarotta, S.; van Bolhuis, F.; Spek, A. L.; Smeets, W. J. J. Inorg. Chem. 1989, 28, 1407. (10) Edema, J. J. H.; Gambarotta, S.; Spek, A. L. Inorg. Chem. 1989, 28, 812.
0020-1669/89/ 1328-3782$01.50/0 0 1989 American Chemical Society
